1. Field of the Invention
The present invention relates to a touch detector that detects key touch states at the keyboard of an electronic musical instrument.
1. Description of the Related Art
Ordinarily, two switches, which are sequentially closed as a key is depressed, are disposed along the line of travel of each key on the keyboard of an electronic musical instrument.
Several well-known conventional methods for employing such switches to determine key depression force, i.e., key depression velocity, that are implemented on electronic musical instruments are:
Method 1. Measurement of the time that elapses between the closing of the first switch and the closing of the second. PA1 Method 2. Compilation of obtained values, as is described in Japanese Examined Publication No. Sho 58-43757, that are calculated, using various performance factors, during the period between the closing of the first switch and the closing of the second. PA1 Method 3. Calculation of exponential function values, as is described in Japanese Unexamined Publication No. Sho 61-41192, during the period between the closing of the first switch and the closing of the second. PA1 a first switch to be closed at a first key displacement position disposed along the line of travel of a depressed key; PA1 a second switch to be closed at a second key displacement position disposed along the line of travel of the depressed key; PA1 key depression detecting means for detecting a closure of the first switch and a closure of the second switch; PA1 touch data computing means for computing touch data for the depressed key during a period that begins following the closure of the first switch and ends with the closure of the second switch; and PA1 delay means for deferring initiation of touch data computation by the touch data computing means until a predetermined time has elapsed following the closure of the first switch.
To determine key touch states, all of these methods initiate touch detection calculation immediately following the closing of the first switch.
With these conventional methods, however, when the scan cycle time for the first and second switches is shortened, so as to attain an optimum detection accuracy, even when a key is depressed at maximum strength the touch data produced will not reflect a maximum value (or a minimum value, depending on value representations). In other words, the elapsed time between the closure of the first switch and the closure of the second switch will never be "0".
When touch detection calculations to determine key touch states are initiated immediately following the closure of the first switch, however, because key mechanical characteristics delay the closure of the second switch a predetermined time, the calculations performed before the second switch is closed are useless for obtaining touch data.
This adversely affects the operation of an apparatus that determines key touch states by calculating exponential function values. Since data for the range wherein the exponential function values reflect the greatest changes are not used, and as data for the range wherein they reflect only slight changes are used, the dynamic range for touch detection is narrowed and touch detection accuracy is degraded. Also, since the calculations performed in the predetermined period immediately following the closure of the first switch are useless, the means employed to perform the calculations, such as a microprocessor, has to carry an unnecessary load.
The above referenced Japanese Examined Publication No. Sho 58-43757 discloses that to acquire key touch characteristics that are suitable for all the keys of a keyboard the rate of change, i.e., the unit time degree of change, of touch data is adjusted in correspondence with the touch requirements of individual keys.
As key touch difference is, however, mainly derived from calculations performed during a predetermined period immediately following the closure of the first switch, it is difficult to obtain a desirable touch characteristic, for a depressed key, by controlling the rate of change of touch data.